Which condition is described as having a very high spongy-to-compact bone ratio and reduced mineralization, commonly affecting the spine, pelvis, femur, and skull?

• A. Paget's disease
• B. Osteoporosis
• C. Osteopetrosis
• D. Rickets

Answer: (A)

Paget disease of bone involves abnormal, excessive remodeling where osteoclasts resorb bone much faster than normal and osteoblasts lay down new bone in a disorganized, rapid fashion. This creates a lot of newly formed bone that is richly trabecular (spongy) and not organized into the usual dense cortex, so the affected bones end up with a higher proportion of spongy bone relative to compact bone. The skull, spine, pelvis, and femur are classic sites because these areas undergo heavy remodeling and bearing forces. The newly formed bone can be weaker and less properly mineralized despite being thickened, which fits the description of reduced mineralization in the context of abnormal, high-turnover bone. This combination—the high spongy-to-compact ratio with poor-quality bone in key axial and proximal long-bone locations—best matches Paget disease.